The invention relates generally to blow molded, non-circular plastic containers.
In the manufacture of blow molded plastic containers for containing liquids such as beverages, it is customary to utilize an injection-molded parison having a threaded finish that forms the threaded finish of the container blown from the parison. The parison may be injection molded from a variety of desirable plastic containers, with a currently particularly preferred material being polyethylene terephthalate (PET).
The configuration and overall aesthetic appearance of a blow molded plastic container affects consumer purchasing decisions. For instance, distorted or otherwise unaesthetic appearing containers may provide the basis for some consumers to purchase a different brand of product which is packaged in an aesthetically pleasing manner.
While a container in its as-designed configuration may provide an appealing appearance when it is initially removed from blow molding machinery, many forces act subsequently on, and alter, the as-designed shape from the time it is blow molded to the time it is placed on a shelf in a store. Plastic containers are particularly susceptible to distortion since they are continually being redesigned in an effort to reduce the amount of plastic required to make the container. This particularly persistent problem in the manufacture of plastic containers is known in the industry as “lightweighting.” Manufacturers continue to develop new technologies that enable them to reduce the amount of PET resin needed to make a bottle without compromising performance. These efforts are extremely important in reducing manufacturing costs because PET resin accounts for a significant portion of the cost of the finished bottle. While there is a savings with respect to material cost, the reduction of plastic can decrease container rigidity and structural integrity.
In the packaging of beverages and other products, especially juice, blow molded plastic PET containers are used in “hot fill” applications, i.e., applications where the blown container is filled with a liquid at a temperature in excess of 180° F. (82° C.), capped immediately after filling, and allowed to cool to ambient temperatures. Internal forces act on the container as a result of the hot fill processing, for example, shrinkage resulting from the cooling of the container contents. Hot fill containers must provide sufficient flexure to compensate for the changes of pressure and temperature, while maintaining structural integrity and aesthetic appearance. Vacuum absorption panels are generally provided in the body of the container to accommodate the internal pressure changes. Hot fill containers molded of PET by this technique have found widespread acceptance in the marketplace.
External forces are also applied to sealed containers as they are packed and shipped. Filled containers are packed in bulk in cardboard boxes, or plastic wrap, or both. A bottom row of packed, filled containers may support several upper tiers of filled containers, and potentially, several upper boxes of filled containers. Therefore, it is important that the container have a top loading capability which is sufficient to prevent distortion from the intended container shape. As containers are lightweighted, external forces such as top loading can act on the weakest structural portion to cause distortion or collapse. This can be include areas that were previously considered structurally sound. This problem is further complicated in non-circular containers.
Typically, a tubular parison is utilized to make circular or other shaped containers. When a circular container is formed from a tubular parison, orientation and stretch levels around the circumference of the container are relatively uniform. However, when a non-circular container is formed from a tubular parison, stretching problems occur during fabrication. Particularly in the base of the container, unequal stretching may result in unequal and not regularly repeatable shrinkage after the tubular parison is stretched into, for example, a square cross-sectional shape. This problematical shrinkage is particularly undesirable in the bottom section of the container at the seating ring and up to the body section of the container, and results in highly stretched corners and less stretched middle sections and sides. This can result in an unstable or tilted container instead one that sits flat upon a shelf or the like, or having visible deformations. Similar though less extreme problems arise in the dome of the container.
Also, when the container is hot filled and sealed, the subsequent thermal contraction of the container tends to deform the container walls and bottom section. Backflow into the filling mechanism and the use of vacuum filling equipment during filling operations can similarly create a partial vacuum inside the container resulting in its deformation. Such deformation typically concentrates at the mechanically weaker portions of the container, such as the unevenly stretched bottom section, resulting in an exaggerated irregular seating surface and commercially unacceptable appearance. This problem is exacerbated when the container body includes collapse panels, indented surfaces areas which provide for controlled, quantified collapse of the container upon evacuation.
By increasing the thickness of the container, it is possible to some extent to strengthen the container and decrease the effects of vacuum deformation. However, as mentioned above, increasing the thickness of the container results in an increase in the amount of raw materials required to produce the container and a decrease in production speed. The resultant increased costs are not acceptable to the container industry. Additionally, even with increased container thickness, there still is uneven stretching around the bottom section of the non-cylindrical container.